Add lint rule labelStyleBigQuery

[k1LoW]

https://cloud.google.com/resource-manager/docs/creating-managing-labels#requirements

[codecov]

Codecov Report

Merging #199 into master will decrease coverage by 0.51%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
- Coverage   67.33%   66.81%   -0.52%     
==========================================
  Files          19       19              
  Lines        3251     3303      +52     
==========================================
+ Hits         2189     2207      +18     
- Misses        832      869      +37     
+ Partials      230      227       -3     

Impacted Files	Coverage Δ
config/lint.go	95.18% <90.38%> (-1.27%)	⬇️
datasource/datasource.go	21.26% <0.00%> (-13.13%)	⬇️

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